The effects of the social environment on health outcomes have been demonstrated in numerous research studies. However, the mechanisms or intermediate variables through which these effects occur are largely unknown. While methods for mediation analysis exist for the study of intermediate variables (or mediators), these methods are inadequate for handling the challenging features of social environment data. The proposed research will develop new mediation analysis methods using a potential outcomes and structural (causal) model framework. Our two-pronged approach, referred to as generalized causal mediation analysis, will provide new methods for both the fitting of causal models and the computation of mediation effects. In particular, we will extend previous methods by handling models with the following features: 1) mixed discrete and continuous variables, 2) multiple exposure variables, 3) heterogeneous mediation effects, 4) multi-level data, and 5) latent variables. Further extending the causal model framework, in which mediation has an intervention interpretation, we will develop a method to predicting the effect of a new intervention. This approach will involve user augmentation of a fitted causal model, thus synthesizing theory and empirical results. The methods will be applied to three motivating datasets: 1) family environment and dental caries data from 224 very low birth weight and normal birth weight adolescents and their parents; 2) data from a study of an intervention to prevent STDs involving 1357 suburban high schools students; and 3) data from a study of oncologist-patient communication and its effect on patient decision satisfaction. The new methods will be evaluated and refined through the data applications and through simulation studies. Finally, the project will develop user-friendly computer programs to allow behavioral and other health researches to apply the new methods. The new methods will allow a more refined, valid, and thorough exploration of mediating pathways for social environment effects. They will also facilitate the formulation of effective behavioral and other interventions, utilizing knowledge about social environment mechanisms, to improve health outcomes.